1. Field of the Invention
This invention relates to tetragonal zirconia containing materials; and more particularly to the a process for producing tetragonal zirconia containing ceramics wherein low temperature degradation is prevented.
2. Description of Prior Art
Ceramic materials used for structural applications are required to exhibit high hardness, strength and fracture toughness. One class of materials meeting these criteria are those containing zirconia as a constituent thereof. Zirconia imparts toughness to a material through a stress-induced phase transition from the metastable tetragonal to the equilibrium monoclinic phase. The transition is accompanied by a volume increase of approx. 5 percent, which changes the stress field around an advancing crack. The energy needed to propagate the crack is increased, therefore increasing the toughness.
In order to retain zirconia is its tetragonal form at room temperature after sintering, stabilizing oxides, such as Y.sub.2 O.sub.3, CeO.sub.2 or MgO, are added in amounts ranging from about 1-10 mol. %. The larger the grain size of the sintered material, the more unstable the tetragonal phase becomes. The stability of the tetragonal phase is the major factor determining the degree of toughening that the sintered zirconia-containing material will achieve. The toughness of such material increases as the stability of the tetragonal phase decreases.
Zirconia ceramics containing yttria as the stabilizing agent have been shown to have the highest strength of any material yet tested. The main drawback of these materials is the difficulty of controlling the metastable nature of the tetragonal phase. The same transformation which imparts high strength and toughness can cause a large reduction in strength when the material is exposed to temperatures in the range of RT to 500.degree. C. for an extended period of time. Several detailed review papers have described the phenomena, and discussed a number of explanations. The exact mechanism for this degradation is reasonably well understood, although there are occasional examples of experimental results that seem not to support these theories. One such example is illustrated by the fact that the degradation can take place at room temperature, given enough time. Y-TZP material having a low (2.5 mol. %) yttria concentration was stored under office conditions for several years, after which spontaneous cracks occurred in a large number of pieces.
The mechanism that results in the strength degradation is well understood. At the surface of the part, the spontaneous transformation from tetragonal to monoclinic begins, and proceeds (perhaps autocatalytically) into the bulk of the material. This transformation causes cracks which obviously reduce the mechanical properties.
Degradation of tetragonal zirconia containing materials can be prevented by decreasing the metastability of the tetragonal phase. This is accomplished by either increasing the yttria content, or decreasing the grain size. Each of these methods has the disadvantage of reducing the high toughness of the material. Another method of preventing degradation of tetragonal zirconia containing materials is to effect change in the surface region as shown by U.S. Pat. No. 4,525,464 by Claussen et. al. The stability of the surface region is increased by increasing the surface stabilizer content. A zone of fully stabilized zirconia is created on the surface by sintering the material in a bed of stabilizing oxide. The stabilization of surfaces by this method is expensive, and is particularly difficult for complex parts or sharp corners.
There remains a need in the art for an economical method that prevents low temperature degradation of zirconia containing materials.